1. Field of the Invention
The present invention relates to a channel equalizer of a single carrier receiver, and more particularly, to a channel equalizer of a single carrier receiver that performs equalization of a received signal with respect to channel environments using a plurality of decoded symbols provided by a trellis decoder, and a channel equalizing method thereof.
2. Description of the Related Art
As for a transmission scheme for digital broadcasting signals, there are mainly a vestigial sideband (VSB) modulation scheme and a coded orthogonal frequency division multiplexing (COFDM) modulation scheme. The VSB modulation scheme transmits broadcasting signals with a single carrier. The COFDM modulation scheme multiplexes and transmits the broadcasting signal through multiple transmission channels. The VSB modulation scheme is a US-oriented digital broadcasting transmission scheme that has been adopted in various countries including South Korea and U.S.A., while the CDFDM modulation scheme is a European-oriented digital broadcasting transmission scheme.
A current standard adopted for the VSB modulation of a US-oriented terrestrial wave digital television is an ATSC-8VSB that converts the broadcasting signal to be transmitted into 8 levels. Meanwhile, a VSB receiver receives the broadcasting signal that has been modulated by the VSB modulation. The VSB receiver is provided with a channel equalizer that equalizes distortions occurring in transmission channels.
FIG. 1 is a view showing a channel equalizer adapted for use in a conventional single carrier receiver by way of an example. The channel equalizer of the single carrier receiver includes a feed-forward filter 10, a feedback filter 30, an adder 40 and a slicer 50. The channel equalizer having the feed-forward filter 10 and the feedback filter 30 is called a decision feedback equalizer (DFE).
The feed-forward filter 10 removes a pre-ghost influence from respective symbols of the broadcasting signal. The feedback filter 30 removes a post-ghost influence from the respective symbols of the broadcasting signal. The adder 40 adds a remnant value obtained from the feed-forward filter 10 after the pre-ghost removal, with another remnant value obtained from the feedback filter 30 after the post-ghost removal.
The slicer 50 determines a level of a signal obtained from the adder 40 to be a nearest one among predetermined levels. The slicer 50 feeds back the determined signal level to the feedback filter 30.
The feed-forward filter 10 includes a buffering unit 12, a multiplier 14, and a second adder 16. The buffering unit 12 stores and buffers broadcasting signals in respective buffers Z−1 in an inputting order and in a symbol unit. The multiplier 14 multiplies the respective symbols that are stored and buffered in the buffers Z−1 of the buffering unit 12 by a feed-forward filter tap coefficient of an equalizer (not shown), thereby removing the pre-ghost from the broadcasting signals. The second adder 16 adds up values obtained from the multiplier 14 after the removal of the pre-ghost from the broadcasting signals.
The feedback filter 30 includes a buffering unit 32, a multiplier 34 and a third adder 36. The buffering unit 32 stores and buffers level data determined at the slicer 50 consecutively in the inputting order and in the symbol unit. The multiplier 34 removes the post-ghost from the respective symbols that are stored and buffered in the respective buffers Z−1 of the buffering unit 32. The adder 36 adds up the values obtained after the removal of the post-ghost from the broadcasting signals.
With the channel equalizer of FIG. 1, the determined level data are input by the slicer 50 as an input of the feedback filter 30. If an error occurs in the slicer 50 in determining the level data, the error level data is passed through the feedback filter 30 and added to an output value from the feed-forward filter 10 at the adder 40.
Meanwhile, the slicer 50 usually has a different performance in determining the level data according to the number of levels set for the broadcasting signal. For example, with respect to the same electric value of the broadcasting signal, an 8-VSB modulation having 8 levels has a gap between the signal levels as half as a 4-VSB modulation having 4 levels. Accordingly, the possibility that the slicer 50 may have an erroneous level data determination increases.
As the error of the slicer 50 increases, an error propagation occurs at the feedback filter 30. Furthermore, a signal to noise ratio (SNR) greatly decreases in accordance with the error of the slicer 50, thereby degrading performance of the channel equalizer and as a whole, degrading the receptivity of the single carrier receiver.
In order to avoid performance deterioration of the channel equalizer due to the erroneous level data determination by the slicer 50, a trellis coded modulation (TCM) may be adapted to perform trellis coding with respect to the signals being input to the channel equalizer.
FIG. 2 is a view showing another channel equalizer adapted for use in the conventional single carrier receiver, i.e., the conventional VSB receiver. As shown in FIG. 2, the channel equalizer includes the feed-forward filter 10, the feedback filter 30, the adder 40, the slicer 50, a computation unit 60 and a trellis coded modulation (TCM) unit 70.
The feed-forward filter 10 removes an influence by the pre-ghost with respect to the respective symbols of the broadcasting signal. The feedback filter 30 removes the post-ghost with respect to the respective symbols of the broadcasting signal. The adder 40 adds the remnant value obtained at the feed-forward filter 10 after pre-ghost removal, to the another remnant value obtained at the feedback filter 30 after post-ghost removal. The slicer 50 decides the level of the signal obtained at the adder 40 to be the nearest one among the predetermined levels.
The computation unit 60 calculates a difference between output values from the adder 40 and the slicer 50. The TCM unit 70 performs trellis decoding with respect to the output value of the adder 40. The TCM unit 70 feeds back a resultant value of the trellis decoding to the feedback filter 30. Accordingly, the feedback filter 30, based on trellis decoded data fed back from the TCM unit 70, removes the post-ghost from the respective symbols of the broadcasting signal.
The trellis-decoded data is, due to a feedback delay caused by the trellis decoding, input to the feedback filter 30 during a predetermined time period in which the feedback filter 30 removes the post-ghost from the respective symbols based on the output value from the slicer 50. Accordingly, the feedback filter 30 has to remove the post-ghost from the respective symbols based on the level data determined at the slicer 50 until the trellis decoded data is input from the TCM unit 70, and this causes the slicer 50 to erroneously determine the level data.
Furthermore, under a channel environment where ghost which is robust to the broadcasting signal exists prior to an output delay of the TCM unit 70, a high equalization performance cannot be guaranteed even from the channel equalizer having the TCM unit 70 employed therein.